In pyrolytic cracking operations, feedstocks such as ethane, propane, naphtha, kerosene, gas oil, fuel oil and the like undergo “cracking,” i.e. the removal of hydrogen, to form unsaturated hydrocarbons. Pyrolytic cracking also tends to produce oxygenated hydrocarbons, including carbonyl compounds such as acetaldehyde. In a typical operation, the cracked effluent stream is quenched and fractionated and compressed. Acidic contaminants such as hydrogen sulfide, carbon dioxide and mercaptans are typically then removed from the effluent hydrocarbon stream by washing in a caustic scrubber.
The caustic scrubber partially removes the oxygenated hydrocarbons. At the same time, however, the basic conditions in the scrubber tend to cause base-induced condensation reactions of the carbonyl compounds, including in particular aldehydes (e.g., acetaldehyde) and/or ketones, which in turn result in the formation of polymers. The polymers deposit and form on the internal surfaces of the scrubber. As the mass of polymer grows, it leads to fouling and can eventually obstruct the flow of liquids through the system. This is undesirable, as the down-time required to remove the deposited polymer and clean the equipment can be significant.
In the past, polymerization of oxygenated compounds, such as carbonyl-containing organics in basic solutions, has been stopped by adding amine compounds such as hydroxylamine hydrochloride, hydroxylamine sulfate, hydrazine, carbohydrazides and the like. Several patents relate to methods of inhibiting carbonyl fouling. But no patent has been reported regarding the dissolution of the polymers once formed.
U.S. Pat. No. 4,673,489 discloses using hydroxylamine and its salts of hydrochloric acid and sulfuric acid to inhibit polymer formation caused by condensation reactions of aldehydes contained in caustic scrubber units. One disadvantage of the method is that the additive has to be used in almost molar proportion.
U.S. Pat. No. 4,952,301 discloses using ethylenediamines with the molecular formula N2(CH2CH2NH)xH where x is an integer ranging from 1 to about 10 to inhibit carbonyl based fouling, particularly aldehyde fouling, that often occurs during caustic scrubbing of liquid or gas phase hydrocarbon streams in the base wash unit.
U.S. Pat. No. 5,264,114 also discloses the use of amine compounds to inhibit the deposition of foulants during caustic washing of the hydrocarbon gases contaminated with the carbonyl compounds which comprises of treating the hydrocarbon gases with an aqueous amine solution wherein the aqueous amine solution comprises water and an amine compound having a concentration range of 2 ppm to about 5000 ppm and wherein the amine compound is selected from group of organic compound of the formula RNH2 and R2NH, wherein R is selected from the group of alkyl or aryl groups.
Carbohydrazide has been disclosed in U.S. Pat. No. 5,160,425 as useful for inhibiting polymeric fouling deposits during the caustic scrubbing of pyrolytically-produced hydrocarbons contaminated with oxygen-containing compounds.
U.S. Pat. No. 5,288,394 describes a method of inhibiting formation of polymeric fouling deposits after caustic scrubbing of a hydrocarbon stream contaminated with oxygenated compounds with a basic washing solution having pH more than 7, comprising adding to the hydrocarbon stream a sufficient amount of a compound that inhibits formation and deposition of fouling materials comprising at least one hydrazide compound.
U.S. Pat. No. 5,194,143, granted to Roling describes and claims a method for inhibiting the formation of polymeric based fouling deposits during the basic washings of olefins containing hydrocarbon contaminated with oxygenated compounds comprising adding to the wash about 1 to 10000 parts pre million acetoacetate ester compound having the formula CH3COCH2CxHy where x is an integer from about 1 to about 8 and y is an integer from about 3 to about 17. U.S. Pat. No. 5,220,104 discloses the use of percarbonate salts for the same purpose.
In U.S. Pat. No. 5,770,041 Lewis et al. describe the use of certain aldehydic compounds without alpha hydrogen atom or non-enolizable aldehydes such as formaldehyde or glyoxal as aldol inhibitors. Relatively large amounts of the inhibitors disclosed in the '041 patent must be used per mole of carbonyl species.
U.S. Pat. No. 5,710,455 discloses the use of certain organic amine inhibitor like sulfanilic acid for inhibiting the aldol condensation but this patent does not disclose the use of said inhibitor for dissolving the polymer once made which is extremely severe fouling conditions.
Finally, amide condensation products of alkylene polyamines with high molecular weight monocarboxylic acids for reducing or preventing the fouling of processing equipment in petroleum or chemical industries are disclosed in U.S. Pat. No. 3,364,130. In the '130 patent, the fouling occurs during heat transfer and is taking place when the system is heated from 200 to 1300° F. The '130 patent does not address fouling that occurs as a result of alkaline conditions. In addition, while the species that cause fouling in '130 is believed to be olefins and dienes; the '130 patent does not address fouling that is derived from carbonyl compounds.
One disadvantage of the prior art systems is that once the acetaldehydes undergo addition reaction to form high molecular weight polymers, the prior art additives are ineffective for removing them. In addition, the prior art additives typically require additive to reactant molar ratios of at least 1:1 for effective performance. The adducts of the high molecular weight polymers with these compounds tend to be insoluble in the basic system. Thus, the prior art additives are ineffective for the purpose of maintaining unobstructed flow through the system.
Another current practice in the industry is to treat the weak caustic with gasoline or another aromatic fraction in order to remove the polymers before sending it to the spent caustic oxidation unit, in order to prevent fouling there. The resulting gasoline-containing streams cause disposal and operational problem, however. Likewise, routing the gasoline-containing stream to other operating units can cause problems due to the presence of the caustic, as it may effect pH, catalyst and the like.
Hence, despite the various advances in the art, it remains desirable to provide an additive that will inhibit polymerization and also dissolve polymers that are already deposited. In addition, the polymeric deposits also cause fouling of downstream units and can pose environment problems of disposal if aromatics are used to remove deposits from the basic wash system. Hence, it is desired to provide a method for preventing fouling that does not create problems in equipment downstream of the scrubber and that does not pose significant environmental problems.